1. Field of the Invention
The present invention relates to a phase grating used to take an X-ray phase contrast image, an imaging system using the phase grating, and an X-ray computer tomography system.
2. Description of the Related Art
Conventionally, an X-ray fluoroscopic technique for using a difference between absorption capacities of X-ray to obtain a contrast image has been studied.
However, the lighter the element is, the smaller is the absorption capacity of the X-ray. Therefore, there is a problem that enough contrast cannot be expected for soft biological tissues and soft materials.
Thus, in recent years, an imaging method for generating contrast based on a phase shift of X-ray is studied.
An example of an imaging method of X-ray phase contrast image (X-ray phase imaging method) using the phase contrast includes an imaging method using a Talbot interferometry.
An outline of the imaging method of the Talbot interferometry will be described with reference to FIG. 4.
In the imaging based on the Talbot interferometry, at least a spatial coherent X-ray source 6, a phase-type diffraction grating (hereinafter, “phase grating”) 1 for periodically modulating the phase of the X-ray, and a detector 9 are necessary.
In the spatially coherent X-ray, the shape of the phase grating 1 is reflected on the X-ray intensity distribution after transmission through the phase grating 1.
In the X-ray intensity distribution, the contrast changes according to the distance from the X-ray source of the X-ray.
The phenomenon that a light/dark periodic image is periodically formed at a specific distance of grating is a Talbot effect. The light/dark periodic image will be called a self-image.
The locations where the periodic intensity patterns image are formed with the highest contrast are determined by the wavelength of the irradiated X-ray or by the pitch of the phase grating 1.
The pitch of the phase grating 1 in the specification denotes a period with aligned gratings.
As illustrated in a schematic diagram of cross section of the phase grating of FIG. 5, the period may be a distance C between center parts of a grating and an adjacent grating or may be a distance C′ between end faces of the gratings.
A structure including structures parallel to each other periodically arranged at constant intervals in FIG. 5 will be called a periodic structure in the present specification.
If a subject 7 is arranged between the X-ray source and the phase grating, the directed X-ray is refracted by the subject 7.
Therefore, an X-ray phase contrast image of the subject 7 can be obtained by detecting the self-image formed by the refracted X-ray after transmission through the subject 7.
However, an X-ray detector 9 with high spatial resolution is required to detect a self-image.
When the X-ray detector 9 with high spatial resolution is not used, the X-ray phase contrast image can be acquired using an absorption grating 8 with enough thickness to provide a high contrast.
The absorption grating 8 is arranged at a location where the self-image is formed. Moire fringes are generated depending on the location relationship between the self-image and the absorption grating 8.
The phase shift resulted from the installation of the subject 7 between the X-ray source and the phase grating can be observed by the detector 9 as a change in the amount of X-ray transmitted through the absorption grating 8 or as a transformation of the moire fringes.
A phase image of the subject obtained by the method is illustrated by 0 to 2π.
If a difference between amounts of phase change is 2πn (n is an integer excluding 0) when X-ray phase images of a plurality of subjects are acquired by the imaging method of the X-ray phase contrast image using X-ray with a single wavelength, the subjects cannot be distinguished.
Therefore, Japanese Patent Application Laid-Open No. 2007-203074 proposes an imaging method of an X-ray phase contrast image using X ray with two different wavelengths as illustrated in FIG. 6.